I think you are looking for Equal Cost Multipathing, just a flow-aware version of it, instead of per-packet. As to when that will be released, you'd have to get in touch with your Juniper SE. Roadmap information is typically under NDA.
Netscreen used to have this option of sending the return traffic back to the mac address where the flow originated from, without consulting the routing table. In the older versions, that even used to be the default behavior. Usually it just caused troubles (especially with hsrp involved) but there are edge cases where this functionality is interesting (especially for VPN traffic).
It doesn't exist in SRX yet and to be honest, this is the first time I see someone ask for it, so I don't expect it to be a high priority feature. But you'll have to talk to the SEs to see if its on the roadmap.
I've also noticed that ScreenOS (at least version 5.4) does not need the route back, and SRX does need it.
But in some cases it is more tricky on SRX: when you forward traffic from one routing table to another (using e.g. "next table" option), the reverse route in the destination routing table is not needed. So the answer may also depend on your particular situation.
You can enable the "per packet" load balance in forwarding optin on SRX. Althought the name is "per packet", it is actually per session. You get the same result with "forward return traffic to the interface it previously came from" on NSM